Method and apparatus for measuring depths



.arch"1 19 36.- E. TUR-NEI'Q, JR I 2,033,160

' METHOD AND APPARATUS FOR MEASURING DEPTHS Filed April 17, 1928 6 Sheets-S heet l Z4 Z5 27 I 2e IN V EN TOR.

faww Z. Tamer J/r ATTOR Y.

March 10; 1936. E. E. TURNER, JR v METHOD AND APPARATUS FOR MEASURING DEPTHS Filed April' 17, 1928 e Sheets-Sheet 2 [N V EN TOR.

- faw/n ff firms/n1 ATTORNEY.

March 10, 1936. TURNER, JR 2,033,160

METHOD AND APPARATUS FOR MEASURING DEPTHS Filed April 17, 1928 s Sheds- Sheet 4' INVENTOR. 5020/)? E firmer d7:

March 10, 1936. E. E. TURNER. JR 2,033,160

METHOD AND APPARATUS FOR MEASURING DEPT HS Filed April 17, 1928 e Sheets-Sheet s Pas/flan. of ger Hg. 1']

IYINVENTOR. H 7 fdw/nf Tar/7e- ./r

March 10, 1936.

E. E. TURNER, JR

METHOD AND APPARATUS-FOR MEASURING DEPTHS Filed April 17, 1928 6 Sheets-Sheet' 6 I /KQ Patented-Mar.- 10, 1936 PATENT OFFICE METHOD AND APPARATUS roa nmascnme DEPTHS Edwin Turner, In, West Roxbury, Masa, as-

signorto Submarine Signal Company, Boston, Masa, a corporation of Maine Application April 1'], 1928, Serial No. 270,660 12 Claims. (cam- 386) The present invention relates to distance finding, and more particularly perhaps, depth finding by thetimeof travel method employing a source of und and a time measuring device for meas- 5 uring the time interval between the emission of the sound signal and the return of the echo from the bottom of the ocean or the reflecting surface. The present system contains many advantages overithe prior art, both in the ease of obtaining lo the desired results with a minimum of apparatus with regard to cost, and also with very great accuracy.

As contrasted with some of the prigr methods of depth sounding, the present systemfiemploys 15 a hammer striker which is designed to have a high damping and high increment of sound amplitude, not so much to prevent continued vibration of the sounder as to provide a device which cooperates with the particular receiving 20 circuit, which alsoresponds to a signal of high increment, as will be explained below. The cir-. cult is similar to the oneshown in my copending United States application Serial No. 220,719, flied September 20, 1927, Patent No. 1,991,430. 25 In prior systems for depth sounding employing the time of travel method it has been common practice to use a sound of sustained vibration and,

to operate therewith a receiving circuit employing vacuum tubes for making or breaking a cur- 30' rent in a discharge indicator. Such a system has been described in the Dorsey Patent No. 1,- 667,540. This system has proved to be very successful'in producing a commercial depth soundin'g apparatus in which the depth is given auto- 35 matically and visually without any adjustment on the part of the'observer. .The observer has only to look at the dialof the indicator and read olf the depth. The chief difilcultywhich has been encountered with this system is the presence 40 of stray signals under certain conditions and the expense of the apparatus due, in part, to the necessity of special means for 'generating current suitable to operate the sound producer. In the present system the applicant has discovered 45 that lie-can concentrate practically the entire "energy of argreat number of vibrations into'asingle impact vibration and, further, that he can receive this type of signal on amicrophone or hydrophone and operate a discharge indicator 50 therewith by means of a special circuit adapted to use efliciently the energy of this single vibration. While the receiving circuit may possibly be tuned, it.wili and: does operate without tuning and efliciently uses the potential difference gen- 55 erated by the single impact in controlling an elec tronic tube circuit for suddenly making or break ing a current to the discharge tube.

The circuit shown in the above mentioned application "operates only on arapid change of current and only once until it is restored. To ac- 'complish this the increment of the signal must be high and similar to that of the receiving circuit. This is provided by a single powerful im- 'pact which has a definite force. This single sound impact, when echoed, acts on the receiving circuit and causes the transient phenomenon a which operates the indicating mechanism.

Not only is it possible to eliminate stray signals, since only this particular type of impact is capable 'of operating the receiving circuit, but it is also possible to obtain more accurate measurements than hertofore since, instead of havingto build up the signal received by one or two oscillations, as was formerly necessary in many cases and perhaps in all cases, the present signal receiver will operate on the single impact, which definitely fixes the time of the receipt of the echo orreceivedsignal.

Besides this, the. present type of mechanism eliminates all need of high frequency generators and elaborate switch-boards to run oscillators so called, which have been used to create sustained but shortvibrations in this work.

The present system is designed chiefly for stability, long life, low cost, fe'w repairs, something which can be installed upon a vessel and run continuously,if desired, by unskilled operators and give excellent results.

Without enumerating all the advantages and features of this new system as they will appear from the description below, I shall now describe the embodiment shown in the drawings, in which:

Figure 1 represents a diagrammatic view of the entire system;

Figure 2 shows a front view of the indicating e and meauring device with the front casing partly removed;

Figure 3 shows a side view of the mechanism of Figure-2,,looking from the left; I

Figure 4 shows a section on the line H of 5 Figure 3 in, the direction indicated;

Figure 5 shows a detail;

a Figure 6 'showsa front view of the indicating mechanism:

Figure '7 shows a vertical section through the striker;

Figure 8 shows a top oi the striker with the outer casing removed;

Figure 9 shows the mounting of the striker;

Figure 1c shaws a detail of the mounting: and

Figure 11 illustrates a feature of the design of the striker.

In-Figure 1, l is a hydrophone or other sound receiver capable of receiving the sound impulse while the other lead 2| is connected through an adjustable direct current source IS in the following manner. The battery l3, when the switch I! is closed, is connected across the potentiometer l2, one side of the potentiometer and the battery being electrically connected to the hydrophone terminals 2|. The potentiometer arm I 9 is connected to the other terminal 23 of the transformer 3. This arm is also shunted by a condenser 2 to the terminal 2| so that any pulsating variations due to the moving of the potentiometer arm l9 back and forth are not reproduced or indicated upon the indicator 1 6. The use of the potentiometer in this connection allows a careful adjustment of microphone current, which is found very advantageous in getting just the right current for operating the receiving circuit.

The secondary of the transformer 3 has one lead connecting directly tothe grid of the vacuum tube 1, while the other connects to the filament of thesame tube through the battery 4, which places a negativepotential upon thegrid withrespect to the filament. The plate of the tube I is maintained at a positive potential with respect to the filament by means of the battery 6. which has its positive side connected to the plate and its negative side connected to the filament through the resistance Sand condenser I connected in parallel. The negative side of the battery 6 is also connected with the grid of the tube 8, while the filaments of the two tubes, it will be,5noted. are connected in series with the resistances l4 and I5, across the direct current power lines. The potential across the resistance I 4 serves to make the plate of the tube 8 positive by that amount with respect to-the filament. The transformer II is connected in the plate circuit of the tube 8 and servesas the output transformer for the circuit. The primary of this transformer is connected by the leads 24 and 25 to the contacts 26 and 21 respectively and is shorted as shown in Figure 1 for a certain period in the revolution of the cam 28, which is at all times except after the sound has been emitted.

The operation of the circuit is the same as that of the operation of the'circuit shown in my copending application, Serial No. 220,719, mentioned above. I When the impulse is received by the receiver 1, it is transmitted through the transformer 3 and ade to impress a potential upon the grid of the tube I. When this potential is in the right direction, which in the present circuit is positive, the plate filament current in the tube I will increase, and consequently the potential of the point B will drop below the potential of the point A. The grid of the tube 8, which up to this point was the samepotential as the filament, will now become negative with respect to it, and if this change is sharp and of the desired intensity, it will instantaneously cut off the plate current in the tube 8.

rapid breaking or reducing of the'plate current in the tube 8 will cause a large voltage across the transformer II and compel a breakdown across the indicator l6 which is connected across the secondary of the transformer II, T e

indicator I6 is preferably a neon tube which gives an instantaneous glow when an electrical discharge takes place across the electrodes in the tube.

Not every type of signal will operate the indicator, because the receiving circuit used is such that-the signal must have a steep increment in order to effect a rapid enough change to induce the proper voltage in the brief time of the taking place of the phenomenon. In the present. case, unless the change of potential on the grid of this tube 1 is rapid, the change in the plate current of the tube .1 will not be rapid and as a result the charge on the condenser I0. may begin to leak off before the phenomenon is complete. The design of the circuit is such that unless the change in the plate current of the tube 'l'is very rapid, the potential of the point B with respect to the point A will not decrease sufficiently or sufiiciently quickly to cut oil or induce a large potential across the transformer 'l I.

One of the remarkable features of the circuit is that it may be so designed and adjusted that its increment corresponds to that of the sounder and that water noises or other signals will not cause an indication. Whereas in other depth sounding systems there are frequently many strays present, in the present invention this is practically eliminated, since a sound which does not have the same sharpness or increment as the one for which the circuit is designed will not be received.

The sounder used in the present system is of the impact type and produces a signal in which all of the energy is confined in substantially one vibration. To this extent the impact is not a tuned or a periodic vibration and is not, therefore, as in many of the systems previously used a musical note. The receiving cirnoise will not affect the output circuit of this tube whereas the signal potential which is greater than the water noise produces a positive potential on the grid of the tube 1 and causes a rapid increase of platecurrent which is reflected to the grid of the tube 8 causing a breakdown in the plate current of that tube. It should be noted that the receiving circuit depends for its operation upon the potential of the grid of the tube 1 which may be and is established in the present circuit by a single sound vibration and not by a number of vibrations as in other types of circuits and upon the steepness of the impulse wave. This comes from the fact that the output circuit is potentially operated and that the rapidity of cut ofl of the plate current in the tube 8 and in the transformer II is dependent upon the rapidity of the increase of negative potential on the grid of the tube 8. The rapidity of this change is dependent upon the change of platecurrent in the tube 1 and this in turn is dependent upon the rate of change of potential on the grid of the tube 1 which is dependentupon the-steepness of the signal wave and its amplitude. Unless the amplitude of the signal wave impresses a positive potential on the grid of the tube 1 suflicient to increase the potential of. the. grid above the. cut off of the plate current, nothing will take place,

' nal to the rotating disc.

grounded to the disc, which is grounded to the An added featurein the present combination.

is that the receipt of the signal is one of a single phenomenon and not a steady state condition. That is, the first impulse operates the receiving circuit, if at all, and if it does, the second vibration cannot'operate the circuit, since'it takes time for the circuit to recover after operation..

It is known, therefore, within a single vibration between what points the indicator measures. This is borne out by the steadiness with which the indicated signal in actual measurement stays at one place on the scale when the ship is stationary in still water.

' .The indicator is mounted on a rotating disc 30 by means'of a pair of'clips 3| and 32 in a small slot near theedge of the disc. One of the clips 32 serves also as a, brush for conducting the sig- The other clip is frame of the case as indicated by 33in Figure l. The clip and brush 32 bears upon the conducting ring 34, which is fixed and which connects to one. side of the secondary of the transformerl I, the

other side of which is also grounded to the frame.

of the case of the indicator.

The disc 30 is rotated by means of the motor 35, by means of the worm 36 and the gear 31, the latter being mounted and fixed to the shaft 38 carryingthe disc 30. The shaft 38 also carries the gear wheel 39, which meshes with the gear wheel 40. The gear wheel 40 carries a shaft 4|, upon which.is mounted the cams 28 and 43 for controlling the operation of the sounding mechanism. 5

The disc is designed to make three revolutions to one of the cams 28 and 43. The cam 28 con trols the indicator and short circuits it at all times except after the sounder has emitted its signal and until just before the contacts M and 45 close. the disc 3|] to complete a sounding cycle.

- During substantially two revolutions of the indicator it is made non-operative and during these two revolutions the hammer is pulled up and allowed to drop, whereupon the contacts 26 and 21 are opened immediately. The position of the cams shown in Figure 1 is when the striker has sounded and the indicator has not yet been made operative. The indicator is made operative depending upon tl'ie spacing of the sounder and receiver at the moment when the direct signal at the receiver has just passed by. In this manner the effect of the direct signal is entirely eliminated. f

At the moment when the contacts 34 and 35 are closed, if the switch I1 is closed current will be impressedupon the sounder 16 by means of the leads 4! and 48, When these contacts are again open, the hammer 49, which had been previously drawn up, will be released and strike the dia-' phragm 50, imparting through inpact a'vibration to the diaphragm 5|, which may be the skin of a vessel. A condenser is provided for preventing the sparking across the contacts 44 and t5. Themotor35 has its armature connected acrossa potentiometer 52, which in turn is connected across the power supply line 53. In Figures 2,3, 4 and 6 is shown the indicating mechanism. The mechanism is'contained It takes, therefore, three revolutions of within a casing 60, the cover SI of which may be removed by the removal of the machine screws 62, 62, 62, 62-. The cover 6| holds the glass plate and the opening 64 for the reed 66 of the frequency meter, which will be described below.

An opening is provided in the lower part 'of the casing for the three controls which are mounted within the casing. v 4

The cover 6| may in this way be removed and the mechanism emosed without removing any wires or controlling mechanism. In this manner, inspection and repairs-upon a. ship can easily be made by the inspector.

V The mechanism further provides facilities of easy repairs and inspection,

63, which is in front of the graduated disc 65 The back half of the casing has four supports 61 extending from the backof the casing. Upon these supporting arms is mounted a frame 68 carrying the entire mechanism within the casing. The frame 68 has four supporting legs positioned to rest upon the supporting legs 61. Ma-

chine bolts 19 pass through the legs 59 and hold the frame 38 firmly upon the supports 61'. The

frame Elihas cast or made integral with it a hearing II and another bearing '|2 which carry all the rotating mechanism. In the bearing H are two roller or ball bearings 33 and M, in which rest the shaft 38 carrying at one end the worm gear 31 and at the other end the disc 39 and the small gear 39. The motor 35 is mounted on a platform 15 which is supported by 'an arm 16 extending from the frame 68. The end of the armature shaft IT has a worm 36 which meshes with the worm gear 31 to drive the rotating mechanism.

The disc 30 carried on the shaft 38 is provided with radial bars 119 which are permanently magnetized iron or nickel. These bars,- as thedi'sc 33 isrotated, pass beneath the reeds 66, which are the speed of the disc is correct the middle reed each iunedto a particular frequency so that when will vibrate with the largest amplitude. ,If the frequency is slightly ofi from the right one, the

reed either to the left or right of the middle one will vibrate with maximum amplitude, depending upon whether it is higher or lower than it should be.

The gear 39, which is mounted on the shaft 38, meshes with the gear 40, which turns the shaft 6| "carrying the cams 28 and 43. The gearing is V such that the earns 28 and 63 make onervelution to three of the disc 30, thus providing ample time I to draw up the hammer and release it in making the measurement. constructed that the contacts 26 and 2'5 and M and 45 are gradually closed and sharply opened. For this purpose-the cam gradually rises andsuddenly drops after the finger 33 has passed beyond the'point 8|. The contact 231s of the spring type and its tension forces the contact-back assoon as the restraining pressure is released.

The disc 30 is held to the shaft 38 by means of the plate 82, which is pinned at 83 to the shaft. To allow easy adjustment of the disc 33 for-the zero of the scale or to remove it and use another scale, the disc is held to the plate by machine screws 83 which pass through an elongated slot 85 in the disc and hold the disc firmly by means of washers B5. In this way, .if the zero on the scale does not correspond to the zero 'of the indicaor, the scale maybe moved back and forth a bit until the proper zero is obtained.

The earns 28 and 33 are so 7 The scale 65 shown in Figures 2 and 6 is moun'tvedon the supports 81. which are a part of the frame 68, and it is calibrated from zero to the operate. This may vary within a certain range,

but for the most part the system is to be used for depths shallower than 300 fathoms, which depths are usually of greatest interest to ships in indicating the approach of land and guiding in channels and shoal water. v

The construction of the indicating apparatus, it will be noted, allows a mechanic to get at the various elements without great difficulty for inspection and repairs if necessary. No wires come off with the front cover orthe casing, so that there is no danger even if an inexperienced mechanic tried to open the indicator casing.

Besides this, the indicator is easily readable and simply operated, the whole mechanical parts being of much less weight than those heretofore used. If desirable, also, the inside mechanism may be removed and a new one substituted. For this purpose, an inspector may have with him a spareinside mechanism and substitute it if necessary. Or repairs may be made entirely at a shop, the inspector merely replacing the inside mechanism .if it does not work properly. This will always insure operation of the device, besides allowing rapid repair of the system in the event that there is little time before the ship is-leaving.

.A single cable 88 comes through the cable head 8'! and has all the wires for operating the complete system. These wires connect directlyto the switchboard 89 (Fig. 2)-, which is also mounted on the frame 68. All the operator has to do to remove the inside mechanism is to disconnect these wires. 1

The striking mechanism is shown in detail in Figures '7, 8, 9 and 10. It is also shown in Figure 1. It comprises a diaphragm 50, which is held securely against the edge of a casing 99 by means of the machine bolts 9| which pass through a heavy flange 92 at the edge of the diaphragm. Between the casing and the flange 92 is a watertight gasket 93 to keep out the water from the interior of the casing, if necessary, although it is designed to operate in air with the diaphragm imparting the impulse to the ships skin, as will be explained below.

At the center of the diaphragm is a boss 94 which extends on both sides of the diaphragm. On the outside it sets in'a cap 95 which rests, without being rigidly held, against the ships skinv 5|. 0n the'inside it has a flat surface opposing the striker 49, which latter element in its operation hits the boss 94 to impart a sound vibration to the skin 5I and thereby to the water. Beetween the collar 96 and the shoulder of the boss 94 rests a helical spring 91 which is designed so that the forcin helical spring 98 will cause one ,blow to be given to the diaphragm and no more.

The resistance of the spring 91 is just strong enough to prevent a second .blow. The forcing spring 98 rests in a cup 99 at the top of the casing and bears against the striker 49. The spring is centered on the striker by the core I90. The striker 49 rests in ways or grooves IOI and'I92, shown clearly in Figure 8. This maintains it in a central position for-operation.

A laminated core I04, is made up -of the ,laminations I93, which are circular stampings having two radial projectionsto form the poles I I15, I95 across which the magnetic flux flows. The laminations are held together and to the casing by means of the machine bolts I91. Two solenoid coils I05, IIIGare provided for generating this flux. When current is passed through the coils, the space between the poles becomes strongly magnetic and draws up the striker 49 against the spring 98. When the current is released the spring 98 forces the striker down against the boss 94 which imparts a blow. to the 'ships skin 5| and thus to the water. V

The diaphragm 59 is made very thin and the 5 spring 91 just strong enough to prevent a second impact of the striker against the boss 94. The clearance between the striker 49 and the boss 94 is small, yet of suflicient size to allow the striker an acceleration before hitting the diaphragm 50. The diaphragm is not set firmly in the piece 95 .nor isthe latter held firmly against the ships skin 50, so that the impact may be sharp and not opposed before its full force is impressed on the skin 5I. These features allow an impact having a sharp decrement to be obtained.

Another feature of the strike: is illustrated in Figure 11. I In the'design of the ordinary striker, it has been found that the mechanism will operate 20 fairly well for a time and then it will stick. This is due to the change in temperature of the mechanism which causes'the copper to heat and cuts down the effective current. The'magnetization is, therefore, reduced and'whereas initial- 25 ly the curve of the pullof themagnet plotted as ordinate against the position of the plunger is .the curve A, after continuous operation it may be and sometimes is the curve B. The curve representing the resultant force of the spring forces is a straight line obeying Hooke's law and is shown as C. It will benoted that while the curve A is continuously above the line C, the curve 3- drops below it, which means that between E and F the resultant force of the springs is in excess of the 5 magnet pull on the plunger. The magnet cannot pull the plunger up and itwill therefore stick. To avoid this difficulty and to obtain, as well, a constant resultant force, the coil and magnetic circuit is designed so that the flux cur've will al- 40 ways be above saturation, as shownby the curve D. By running above saturation, the force curve becomes a straight line and is everywhere above the resultant spring force, so that the magnet never sticks. This insures reliable and at the same'time accurate operation of the sound producing means.

It is necessary inthe present system to have special'means for. mounting the striking mecha- 95 against the skin of the vessel need adjustment in each case. For this purpose, the sound producer is. provided with four threaded studs I98 which extend from the top of the sound producer casing and which are provided with nuts 55 I09 and III). The studs I08 pass throughv a plate H I which is bolted to the angle iron I I2 by means of the bolts and nuts I I3 and the nuts I09 and I III. The nuts I 99 may be adjusted along the studv to obtain just the right tension of the boss 94 and the piece 95 against theships skin. The nuts I I 0' are brought up tight .with split washers beneath] them, so as to hold the sound producing mechanism rigidly in place. *Two angle irons II 2 are. used to hold the sound producer and thesefare bolted to the frames H4 or otherwise secured impact striker for generating a single sharp sound impulse, a'receiver for receiving said sound 75 2,083,160 impulse after reflection from the object or surface whose depth is to be measured, an indicator, and means for operating said indicator comprising a receiving circuit interposed between said receiver and said indicator and having a. thermionic three electrode control valve, the grid olwhich is provided with a biasing potential adjusted for the operation of the circuit just above the noise level. i

2. A system for measuring depths or distances including an impact striker for generating a single, sharp sound impulse, means for receiving said sound impulse after reflection from the object or surface whose depth is to be measured, an indicator and means for operating said indicator comprising a circuit interposed between the receiver and indicator and having an electronic valve the control grid of which is provided with a biasing potential adjusted for the-operation of the circuit just above the noise level.

' 3. A system for measuring depths or'distances' including an impact striker for generating a single sharp'sound impulse, means for receiving said sound impulse after 'reflection'from the object or surface whose depth is to be measured, an indicator and means for operatingsaid indicator comprising a circuit interposed between the receiver and the indicator, said circuit having two thermionic valves, the first of which has a grid biasing potential set at the level of the noise which is to be eleminated, and an impedance coupling said first valve with the second including a capacity in shunt for limiting the time 6f re-. covery of said circuit.

4. A system for measuring dpths or distances I including an impact striker for generating a single sharp sound impulse, means for receiving said sound impulse after reflection from the object or surface whose depth is to be measured, an electric discharge indicator and an electronic valve circuit interposed between said receiver and said indicator having an initial valve witha grid biasing control potential therefor, a second valve, and means coupling circuits of said valves providing in shunt a capacity in the grid circuit of the second valve for preventing the sudden recovery of said circuit anda repeated discharge of said indicator.

5. A system for measuring depths .or distances including an impact striker for generating a single impulse, means for receiving said sound impulse after reflection from the object whose distance is to be measured, an indicator, means for operating said indicator comprising a circuit in; terposed between the receiver and-indicator and having an electronic valve the control grid of which is provided with a biasing potential adjusted for the operation of the circuit just above the noisejlevel and means for adjusting the sensitivity of the received signal including means for' controlling the polarizing receiver current.

6. In a-system for measuring distance and depth in a field where local noise is generated the combination of means'for producing an impact, microphonic receiving means, an indicator and means for operating said indicator. including an untuned vacuum tube circuit having a grid bias control of a magnitude above that generated by the local noises.

'7. In a system for measuring distance and depth in a field where local noise is generated the combination of means for producing an impact.

means for receiving sound vibrations, an indicator and means for operating said indicator including a vacuum tube circuit having a grid bias control of a magnitude above that generated by the local noises.

8. Ina system depth in a field where local noise is generated the combination of means for producing an impact, means for receiving sound vibrations, an electric discharge indicator and means for operuum tube circuit having a constant plate current flowing in one tube thereof and means forCsud- 'denl'y interrupting said plate current including a second tube for impressing a sudden potential on said first tube, said second-tube having its input connected to said receiver and having a grid biasing potential of a magnitude above that generated by the local noises.

' 9. In a system for measuring distance and depth in a field where local noise is generated the combination of means for producing an impact, means for receiving sound vibrations, a dis-' charge indicator and means for operating said indicator including a vacuum tube circuit having a grid bias control of a magnitude above that generated by the local noise.

10. A system for measuring depths in the presence of extraneous noises which comprises means A j for measuring distance and ating said indicator including an untuned vacfor. generating'a sharp compressional wave im- 7 pulse having an amplitude far above the noise level, means for receiving the; impulse after re-" flection from the object whose distance isto be 'measurcd, a receiving circuit operatively con-' therethrough, means connected with said last means for establishing a variation in current in said operating means said means being coupled with said cathode and said anode and responding to a potential built up by the current therein flowing when the signal impressed on said grid overcomes the point to prevent anode current 11. A system for measuring depths in the presence of extraneous noises which comprises an impact oscillator for producing an impulse hav ing an amplitude far above the noise level, means for receiving tli impulse after reflection from the object whose distance is to be measured, a

receiving circuit operatively connected to said receiver, said circuit including a thermionic valve control means having cathode, anode and grid provided with a negative potential adjusted to the point to prevent anode current flow by action of compressional waves of a magnitude not substantially greater than the extraneous noises, indicating means, means adapted topperate said indicating means only upon a sudden variation of current therethrough, means connected with said last means for. establishing a variation in current in said operating means, said means being coupled with said cathode and said anode and responding to a potential built up by the current therein flowing when the signal impressed on said grid overcomes the point to prevent anode current flow. 7

12. A system formeasuring depths in the presence of extraneous noises which comprises means:

pulse having an amplitude far above the noise level, means for receiving the impulse after reflection from the object whose distance is to be measured, an untuned thermionic tube and circuit having cathode, anode and grid, the grid circuit thereof being directly coupled with said receiver and having a. potential bias adjusted to the point to prevent anode current flow by action of compressional waves of a magnitude-not substantially greater than the extraneous noises, indicating means, a potential transformer adapted to operate said'indicating means upon a sudden variation of current therethrough,-a vacuum tube and circuit in the output of which said transformer is coupled, means directly coupling in the input of said vacuum tube circuit said anode and cathode whereby a current flowing from said 

